Message-Id: <DECRIHBXOEXY.WX45FITGF5DA@bootlin.com>
Date: Wed, 19 Nov 2025 16:05:05 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Louis Chauvet" <louis.chauvet@...tlin.com>, "Andrzej Hajda"
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Cc: "Hui Pu" <Hui.Pu@...ealthcare.com>, "Thomas Petazzoni"
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Subject: Re: [PATCH 06/26] drm/bridge: add devm_drm_of_find_bridge

Hi Louis,

On Wed Nov 19, 2025 at 3:33 PM CET, Louis Chauvet wrote:
>
>
> On 11/19/25 13:05, Luca Ceresoli wrote:
>> Several drivers (about 20) follow the same pattern:
>>
>>   1. get a pointer to a bridge (typically the next bridge in the chain) by
>>      calling of_drm_find_bridge()
>>   2. store the returned pointer in the private driver data, keep it until
>>      driver .remove
>>   3. dereference the pointer at attach time and possibly at other times
>>
>> of_drm_find_bridge() is now deprecated because it does not increment the
>> refcount and should be replaced with drm_of_find_bridge() +
>> drm_bridge_put().
>>
>> However some of those drivers have a complex code flow and adding a
>> drm_bridge_put() call in all the appropriate locations is error-prone,
>> leads to ugly and more complex code, and can lead to errors over time with
>> code flow changes.
>>
>> To handle all those drivers in a straightforward way, add a devm variant of
>> drm_of_find_bridge() that adds a devm action to invoke drm_bridge_put()
>> when the said driver is removed. This allows all those drivers to put the
>> reference automatically and safely with a one line change:
>>
>>    - priv->next_bridge = of_drm_find_bridge(remote_np);
>>    + priv->next_bridge = devm_drm_of_find_bridge(dev, remote_np);
>>
>> Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
>> ---
>>   drivers/gpu/drm/drm_bridge.c | 30 ++++++++++++++++++++++++++++++
>>   include/drm/drm_bridge.h     |  5 +++++
>>   2 files changed, 35 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
>> index 09ad825f9cb8..c7baafbe5695 100644
>> --- a/drivers/gpu/drm/drm_bridge.c
>> +++ b/drivers/gpu/drm/drm_bridge.c
>> @@ -1446,6 +1446,36 @@ struct drm_bridge *drm_of_find_bridge(struct device_node *np)
>>   }
>>   EXPORT_SYMBOL(drm_of_find_bridge);
>>
>> +/**
>> + * devm_drm_of_find_bridge - find the bridge corresponding to the device
>> + *			     node in the global bridge list and add a devm
>> + *			     action to put it
>> + *
>> + * @dev: device requesting the bridge
>> + * @np: device node
>> + *
>> + * On success the returned bridge refcount is incremented, and a devm
>> + * action is added to call drm_bridge_put() when @dev is removed. So the
>> + * caller does not have to put the returned bridge explicitly.
>> + *
>> + * RETURNS:
>> + * drm_bridge control struct on success, NULL on failure
>
> I am not sure for the "NULL on failure", you return ERR_PTR(err), which
> is probably not NULL but an error code.

Indeed.

Apologies for the mess in this series: it was adapted from an old one using
a different approach, so I had to adapt lots of details, and missed a few
along the way. :(

About the value to return, maybe it's better to use the same semantics as
drm_of_find_bridge(), i.e. NULL on error. I don't think a caller would have
anything clever to do with an error return value other tan bailing out. And
the only error path for devm_add_action_or_reset() is on a small
allocation, so it basically cannot happen.

>> +struct drm_bridge *devm_drm_of_find_bridge(struct device *dev, struct device_node *np)
>> +{
>> +	struct drm_bridge *bridge = drm_of_find_bridge(np);
>> +
>> +	if (bridge) {
>> +		int err = devm_add_action_or_reset(dev, drm_bridge_put_void, bridge);
>> +
>> +		if (err)
>> +			return ERR_PTR(err);
>> +	}

So this would become:

	if (bridge) {
		if (devm_add_action_or_reset(dev, drm_bridge_put_void, bridge))
			return NULL;
	}

>> +
>> +	return bridge;
>> +}

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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